Bug 1533617 part 5. Disallow virtual MOZ_CAN_RUN_SCRIPT methods overriding a non-MOZ_CAN_RUN_SCRIPT superclass method. r=andi
This way if a caller calls a method that has a MOZ_CAN_RUN_SCRIPT override, it
can detect that it's possibly calling a MOZ_CAN_RUN_SCRIPT thing without having
to know about the override.
Differential Revision: https://phabricator.services.mozilla.com/D22839